In previous radar systems for small ships, radar data has been displayed in real-time. A problem arose since when radar return signals are processed on a real-time basis, the writing rate of the display cathode-ray tube beam is inversely proportional to the radar range setting and, hence, the display brightness varies with range setting. For the shorter radar range settings, the writing rate upon the display screen became so great that the phosphor on the screen did not receive sufficient electron beam energy during the sweep of the beam to produce sufficient light output to overcome the background ambient light. The lighting problem frequently made these radars difficult to use on boat and airborne radars, both applications which typically are subject to high ambient lighting.
Attempts to solve these problems include those in which the data to be displayed was written first in real-time upon a storage tube and then read out and displayed upon a cathode-ray tube. The reading out from the storage tube took place at a slower rate than the rate at which the pattern was written into the storage tube. These systems suffer from a number of inherent problems. Such systems are costly in that two separate deflection and cathode-ray tube systems must be provided in each radar display. Secondly, performance was degraded below that which could be obtained with only a single tube in that additional noise and loss was introduced with the second tube system.
Another major difficulty with previous systems was that when the range setting was changed, the deflection waveforms also had to be changed to accommodate the sweep time required for the particular range setting chosen. At short ranges, the sweep waveforms into the deflection coils of the cathode-ray tube display were quite short. Consequently, in order to move the beam from the center of the screen to the edge of the screen in the required time took high values of the rate of change of the current in the deflection coils. This, in turn, induced high voltages into the deflection circuitry and made the deflection circuitry difficult and expensive to construct. Also, the fact that the beam deflection time changed for each range setting made it necessary to construct deflection circuits which had a broad frequency range of operation. Moreover, the bandwidth of the deflection amplifiers had to be greater for the short ranges than for the long ranges.
In a preferred embodiment, digital representations or samples of a radar return signal are written into storing means in a first time period and read out in a second time period, the second time period being greater than the first time period for at least some ranges of a radar range setting. Additionally, the first time period may be proportional to the radar range setting while the second time period remains constant. Clocking means which supplies timing pulses to the storage means may be used for determining the first and second time periods. For generating writing clock pulses, a continuously cycling binary counter is preferably used wherein one of the outputs of the counter is selected by the range switch as the source of writing, timing or clocking pulses.
The present invention may also be practical with the method of transmitting and then receiving radar signals, converting received signals to digital representations thereof storing at least a portion of those representations at a first rate, reading out the representations at a second rate slower than the first rate, and displaying data in response to the read out radar return signals.